There exist two commonly implemented front-end architectures in radio frequency (RF) receiver design; namely, the homodyne architecture and the heterodyne architecture. The homodyne architecture down-converts a desired channel directly from RF to baseband, whereas the heterodyne architecture down-converts a desired channel to one or more intermediate frequencies (IFs) before down-conversion to baseband. In general, each of these front-end architectures often employ an antenna to receive an RF signal, a band-pass filter to suppress out-of-band interferers in the received RF signal, a low noise amplifier (LNA) to provide gain to the filtered RF signal, and one or more down-conversion stages.
Each component in a receiver front-end, such as those mentioned above, contributes noise to the overall system. The noise of a component can be characterized by its noise factor (F), which is given by the ratio of the signal-to-noise ratio (SNR) at the input of the component to the SNR at the output of the component:FCOMPONENT=SNRIN/SNROUT The noise of the overall receiver front-end increases from input to output as noise from successive components compound. In general, the overall noise factor of the receiver front-end is proportional to the sum of each component's noise factor divided by the cascaded gain of preceding components and is given by:
      F    TOTAL    =            F      1        +                            F                      2            -            1                          -        1                    A        1              +                            F                      3            -            1                          -        1                              A          1                ⁢                  A          2                      +    …    +                            F                      n            -            1                          -        1                              A          1                ⁢                  A          2                ⁢                                  ⁢        …        ⁢                                  ⁢                  A                      n            -            1                              where Fn and An represent the noise factor and gain of the nth component in the receiver front-end, respectively. The above equation reveals that the noise factor F1 and gain A1 of the first gain component can have a dominant effect on the overall noise factor of the receiver front-end, since the noise contributed by each successive component is diminished by the cascaded gain of the components that precede it.
To provide adequate sensitivity, therefore, it is often important to keep the noise factor F1 low and the gain A1 high of the first gain component in the receiver front-end. The sensitivity of the receiver front-end determines the minimum signal level that can be detected and is limited by the overall noise factor of the receiver front-end. Thus, in many receiver designs the first gain component in the front-end is an LNA, which can provide high gain, while contributing low noise to the overall RF receiver.
LNAs provide relatively linear gain for small signal inputs. However, for sufficiently large input signals, LNAs can exhibit non-linear behavior in the form of gain compression; that is, for sufficiently large input signals, the gain of the LNA approaches zero. LNA gain compression is a common issue confronted in RF receiver design because large out-of-band interferers referred to as blockers can accompany a comparatively weak desired signal in a received RF signal. If these large out-of-band interferers are not attenuated prior to reaching the LNA, they can seriously affect the linearity of the LNA and degrade the sensitivity of the receiver front-end.
Therefore, a band-pass filter is conventionally employed in the receiver front-end, before the LNA, to attenuate large out-of-band interferers. These filters are typically mechanically-resonant devices, such as surface acoustic wave (SAW) filters, that provide a high quality factor (Q-factor) required by many of today's communication standards. The Q-factor of a tuned circuit, such as a band-pass filter, is the ratio of its resonant frequency (or center frequency) to its 3 dB frequency bandwidth. SAW filters are generally not amenable to monolithic integration on a semiconductor substrate with the RF receiver. However, SAW filters remain conventional in many RF receiver designs because of the limited Q-factor of silicon-based inductors.
Although SAW filters can provide excellent attenuation of large out-of-band interferers and accurate pass-band location, they have several associated disadvantages. First, these filters have an approximate insertion loss of 1-2 dB in their pass-band. This directly adds to the noise factor and degrades sensitivity of the RF receiver. Second, these filters invariably add cost and circuit board area, especially in multi-band applications where several of these filters can be required (e.g., one for each supported band). Finally, the use of narrow-band off-chip SAW filters is not compatible with the concept of software-defined radios (SDRs), which continue to generate considerable interest for their associated advantages in power, speed, and flexibility.
For example, mobile applications such as the cellular standards GSM, UMTS, and LTE, the wireless networking standards Wi-Fi and WiMAX, the mobile TV standard DVB-H, the navigation standard GPS, and the short-range communication standards Bluetooth and RFID, generally use different portions of the radio spectrum that span from 400 MHz all the way to 6 GHz. The trend in mobile devices has been, and continues to be, to combine many (if not all) of these mobile applications into a single mobile device. Rather than adding a separate receiver front-end for every one of these applications, the use of flexible receiver hardware controlled by software can make the mobile device, smaller, more power efficient, and cheaper. This trend of moving functionality into software is the basic idea of SDRs. Saw filters are not compatible with the concept of SDRs because they are narrow-band and their pass-bands are generally not programmable.
The embodiments of the present disclosure will be described with reference to the accompanying drawings. The drawing in which an element first appears is typically indicated by the leftmost digit(s) in the corresponding reference number.